River in western Beleriand, which runs close to Nargothrond. It provided a natural defence for this city against the forces of Morgoth, which mostly fearwater. That is, until Turin foolishly advised the elves to build a bridge, which provided a handy access route for Glaurung...

It's featured in Tolkien's book The Silmarillion - see the map at the back for exact details of where it goes.